1. Field of the Invention
The present invention is directed to voltage regulation in multivoltage supply applications, and in particular, for peripheral devices such as memory systems.
2. Description of the Related Art
Non-volatile semiconductor memory is popular for a number of uses, including cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices, audio and video players and other appliances. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memory types.
Typical applications include the provision of one more arrays of storage elements in a memory card system, such as a memory card, which is then inserted in a host device. Both EEPROM and flash memory utilize arrays of storage elements to store one or more bits of data per element. Each element generally includes a floating gate that is positioned above and insulated from a channel region and a semiconductor substrate. The floating gate is positioned between source and drain regions. A control gate may be provided over and insulated from a floating gate. The threshold voltage of each memory transistor is controlled by the amount of charge that has retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before a transistor is turned on to permit conduction between its source entering is controlled by the level of charge on the floating gate. Many EEPROMs and flash memories have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states. Such memory cells store one byte of data. Other EEPROMs and flash memory cells store multiple ranges of charge and, therefore, such memory cells can be programmed to store multiple states. Such memory cells store multiple bytes of data.
Improvements in processing technology have led to smaller transistor device sizes. This in turn has led to lower power supply requirements in each successive generation of transistor technology. For example, a 0.25 micron process device operates on a voltage of approximately 2.5 volts, a 0.18 micron process uses a 1.8 v (+/−10%) supply, a 0.15 micron process will use a 1.5 (+/−10%) volt supply, 0.13 micron technology a 1.2 v (+/−10%) supply, and so on.
Unfortunately, not all host devices supply the requisite power required by such technology. For example, older hosts may supply 3 volts to a memory system requiring only 1.8 volts. Thus, memory devices constructed with newer technologies must allow for the possibility that the host device in which they are used will supply a higher voltage used with legacy products. Hence power level detection and voltage regulation techniques are used to provide the correct voltage to the memory device.
Another consideration is that most regulators impose a significant voltage drop when a substantial current is drawn from the device. A voltage regulator designed to step down a legacy voltage will experience a current drop if a lower than expected voltage is supplied at the input. For example, in a conventional voltage regulator, a 3 volt host has sufficient head room to supply high current to a 1.8 volt device, but a 1.8 volt host can not supply high current with low drop to a 1.8 volt device; the output would be too low.